New Zealand Journal of Geology & Geophysics abstracts

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Methane seepage and its relation to slumping and gas hydrate at the Hikurangi margin, New Zealand

Kevin Faure¹
Jens Greinert^2,3
Ingo A. Pecher³
Ian J. Graham³
Gary J. Massoth³
Cornel E. J. de Ronde³
Ian C. Wright⁴
Edward T. Baker⁵
Eric J. Olson⁶

¹GNS Science
National Isotope Centre
PO Box 31312
Lower Hutt 5040, New Zealand
k.faure@gns.cri.nz

²Renard Centre of Marine Geology (RCMG)
Ghent University
Krijgslaan 281 s.8
B-9000 Gent, Belgium

³GNS Science
PO Box 30368
Lower Hutt 5040, New Zealand

⁴National Institute of Water and Atmospheric Research
PO Box 14901
Wellington 6241, New Zealand

⁵NOAA/PMEL
7600 Sand Point Way NE
Seattle WA 98115-0070, USA

⁶School of Oceanography
University of Washington
Seattle WA 98195, USA

Abstract   Dissolved methane and high resolution bathymetry surveys were conducted over the Rock Garden region of Ritchie Ridge, along the Hikurangi margin, eastern New Zealand. Multibeam bathymetry reveals two prominent, northeast trending ridges, parallel to subduction along the margin, that are steep sided and extensively slumped. Elevated concentrations of methane (up to 10 nM, 10× background) within the water column are associated with a slump structure at the southern end of Eastern Rock Garden. The anomalous methane concentrations were detected by a methane sensor (METS) attached to a conductivity-temperature-depth-optical backscatter device (CTDO) and are associated with elevated light scattering and flare-shaped backscatter signals revealed by the ship’s echo sounder. Increased particulate matter in the water column, possibly related to the seepage and/or higher rates of erosion near slump structures, is considered to be the cause of the increased light scattering, rather than bubbles in the water column. Methane concentrations calculated from the METS are in good agreement with concentrations measured by gas chromatography in water samples collected at the same time. However, there is a c. 20 min (c. 900 m) delay in the METS signal reaching maximum CH₄ concentrations. The maximum methane concentration occurs near the plateau of Eastern Rock Garden close to the edge of a slump, at 610 m below sea level (mbsl). This is close to the depth (c. 630 mbsl) where a bottom simulating reflector (BSR) pinches out at the seafloor. Fluctuating water temperatures observed in previous studies indicate that the stability zone for pure methane hydrate in the ocean varies between 630 and 710 mbsl. However, based on calculations of the geothermal gradients from BSRs, we suggest gas hydrate in the study area to be more stable than hydrate from pure methane in sea water, moving the phase boundary in the ocean upward. Small fractions of additional higher order hydrocarbon gases are the most likely cause for increased hydrate stability. Relatively high methane concentrations have been measured down to c. 1000 mbsl, most likely in response to sediment slumping caused by gas hydrate destabilisation of the sediments and/or marking seepage through the gas hydrate zone.

Keywords   Hikurangi margin; Ritchie Ridge; methane; hydrate; multibeam bathymetry; slumping

G06014; Online publication date 30 November 2006; Received 16 June 2006; accepted 24 November 2006

New Zealand Journal of Geology & Geophysics, 2006, Vol. 49: 503–516
0028–8306/06/4904–0503   © The Royal Society of New Zealand 2006

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